Is this the most loaded, uninformed question on this site? Check the statutory requirements for a "life" sentence state by state. Some require 10 and out, some 15, and others no more than 20 for the same reasons the Brits do. Consider also that there are over 2.5 million people currently incarcerated in the US today and that 98% of those will be released at some time.
Suggestion: volunteer with a jail or prison outreach program and get to know a few incarcerated people. You will find out that they are not all that different than you, with the exception that they made a mistake AND got caught.
And, in case you haven't noticed, the crime rate is growning here. The US Department of Justice itself has published findings that show there is no correlation between the length of incarceration and deterence to crime. There is a direct correlation between longer sentences and recidivism (repeat offenses).
Opinions are fine, but do your homework.